Multi-species coupling mechanism during chloride penetration in cementitious materials: comparative study of Gauss’s law and current conservation methods

The accurate prediction of chloride ingress in concrete is essential for evaluating the durability and service lives of marine and salt-laden concrete structures. Ion–ion interactions (multi-species coupling (MSC)) in the pore solution significantly influences chloride ingress. In this study, three numerical models (based on the Poisson equation, the current conservation equation or the Laplace equation) were developed to simulate the transport of four main ionic species (K⁺, Na⁺, Cl⁻ and OH⁻) in concrete. The transport behaviours of different ionic species and the electrostatic potential obtained from the three models were analysed and compared. The results showed that the influence of MSC on chloride diffusion without external fields is minimal, and the Laplace equation provides a simple and accurate approximation in this case. By contrast, when an external voltage is applied, MSC arouses obvious non-linear electric fields, accelerates chloride ingress and aligns the migration wave fronts of ions with the same charge. The Poisson and the weak electroneutral (Weak-EN) models produce nearly identical ionic distributions, indicating both can capture ion–ion interactions. The Poisson model was found to be is more suitable for incorporating chemical reactions and other physicochemical processes, while the Weak-EN model offers a balance between accuracy and computational efficiency. These findings clarify the applicability and limitations of different modelling approaches, providing guidance for selecting appropriate methods in analysis of concrete durability.